Conventionally, a power generator such as a solar cell or a fuel cell is installed in a consumer's home so that the power generator covers a part of the necessary electric power in the consumer's home. For example, in Japan, if the power generator is a solar cell, when the surplus electric power is generated (the generated electric power by the solar cell is more than the consumed electric power in a load device), the surplus electric power by the solar cell is generally caused to reversely flow to a commercial power system and is sold to an electric power company.
Further, with a further widespread use of solar cells from now on, there is a view that power selling is to be regulated from the viewpoint of stabilizing the electric power in commercial power systems. It is considered that the surplus electric power generated by consumers cannot be freely sold from now on. Moreover, it is also considered that a significant drop of a unit price of the power selling reduces a merit by the power selling. In such a case, it is possible to install a storage battery in a consumer's home to store the surplus electric power therein (power storage). However, there is a problem in that a burden on the consumer due to the introduction of the storage battery is increased or if the storage battery is fully charged, the electric power cannot be stored any more.
With regard to the problem, as a system which effectively utilizes the surplus electric power other than a method of the power selling or the power storage, proposed is a system in which the surplus electric power is used for an operation of a (hot water storage type) hot water supply device (see, Japanese Patent Application Publication 2006-158027). In the system, if the generated electric power possibly has the surplus electric power, a value of the minimum amount of thermal storage of the hot water supply device is increased. In this time, if the actual amount of thermal storage of the hot water supply device is less than the minimum amount of thermal storage, the hot water supply device is operated with the generated electric power by a power generator to boil additional hot water.
However, when the surplus electric power is used for an operation of a certain load device (for example, hot water supply device) as described above, if the surplus electric power is run out after the load device starts a certain process and before the process is completed, the following problem might be caused.
Specifically, in a case where a hot water supply device starts a process of boiling a predetermined amount of hot water, if the surplus electric power is run out before completion of the process (completion of boiling), the process of boiling hot water is finished in the course thereof. As a result, the hot water supply device repeatedly starts and finishes the process every time the surplus electric power is generated. This results in the worsened utilization efficiency of the surplus electric power, compared with a case where the same process is completed at one time.
Alternatively, it can be considered that when the surplus electric power is run out before completion of the process, the hot water supply device receives electric power supply from a commercial power system in order to continue the process. However, regardless of starting boiling hot water for utilizing the surplus electric power, a consumer is forced to purchase the electric power eventually. Accordingly, in some cases, a process (hot water boiling), which may be originally executed at midnight when a unit price of power purchasing is cheap is started during the daytime. This causes a consumer to be forced to purchase the electric power during the daytime when the unit price is expensive, and thus the consumer might experience a financial loss.